Roll a version that cannot start back without waiting for a click.

start.pyw showed the import failure in a message box before it ended with
code 86, and the stub waits six seconds for that code: unless the player
closed the box in time, the broken version stayed the current one. When
app/previous.txt names another complete release, the failure now goes to
crash.log only and the process ends at once, so the stub rolls back and
the previous version says on its home screen which version did not start.
The box stays when there is nothing to go back to, when Steam started the
launcher without the stub, and when the stub's wait is already over.

Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>

docs/launcher.md

@@ -254,7 +254,10 @@ The stub swaps `python.next` in while no Python runs, then starts the new versio
    one.
 
 When the new version fails to start (exit code 86 from `start.pyw`, or any error within six seconds), the stub sets
-`current.txt` back to `previous.txt`'s version and starts it with `--rolled-back`. The game is never rebuilt while it
+`current.txt` back to `previous.txt`'s version and starts it with `--rolled-back`. `start.pyw` writes the failure to
+`data/logs/crash.log` and shows it in a message box only when there is no version to go back to, when Steam started
+it, or when the six seconds are over (`quiet_exit`): a box holds the exit code back until it is closed, and the
+broken version then stayed the current one. The game is never rebuilt while it
 runs. Keep Build Cache off deletes the lifted C and the objects (about 5 GB) after each build but keeps
 `build-cache/state.json`, so updates still rebuild without the dump (compiling everything).